As an optical head device of the prior art, an example of an in-line differential push-pull method (DPP method) has been available in which the region of a diffraction grating is divided into three and the phase of diffracted light is set at −90°, 0°, and +90° in the respective regions (for example, see Japanese Patent Laid-Open No. 2006-179184 (for example, see FIG. 7)). By providing the three regions, it is possible to achieve the effect of suppressing an amplitude change of a tracking error signal even when an objective lens moves in the track traversing direction (a shift of the objective lens). FIG. 19 shows the configuration of an optical head device 100 of the prior art which is described in Japanese Patent Laid-Open No. 2006-179184.
Referring to FIG. 19, the following will describe the configuration and operations of the optical head device 100 according to the prior art.
A light beam emitted from a semiconductor laser 101 generates ±first-order diffracted light (not shown), which acts as a sub beam of the in-line differential push-pull method, through a diffraction grating 102 having divided regions. The light beam having passed through the diffraction grating 102 is reflected in a beam splitter 103 and then is collimated through a collimator lens 104. The collimated light beam is circularly polarized through a λ/4 wave plate 105, is incident on an objective lens 106, and becomes convergent light. The convergent light is emitted to an optical disk 107. The light reflected and diffracted by an information layer of the optical disk 107 passes through the objective lens 106 again and then passes through the λ/4 wave plate 105 and the collimator lens 104. After that, the light transmits through the beam splitter 103. The objective lens 106 is moved in a direction along an optical axis and a direction perpendicular to a track by an actuator 109. The light beam having transmitted through the beam splitter 103 passes through a detection lens 110 and is incident on a photodetector 111.
FIG. 20 is a front view showing a state of the divided regions of the diffraction grating 102. The diffraction grating 102 is made up of three regions 121, 122, and 123. A circle 130 in FIG. 20 is formed by projecting a light beam, which is incident into the objective lens 106, onto the diffraction grating 102 when the objective lens 106 focuses on the information layer 108 of the optical disk 107. On the regions 121, 122, and 123, grooves are formed at predetermined periods. Although the grooves are evenly spaced in the respective regions, the peaks and valleys of the grooves are each shifted in phase by 90°. In other words, when the region 122 has a phase of 0°, the region 121 has a phase of −90° and the region 123 has a phase of +90°.
Thus it is possible to add a predetermined wave front to ±first-order light acting as a sub beam. Further, when the sub beam is located on the same track as convergent zeroth-order light acting as a main beam, it is possible to obtain from the sub beam a tracking error signal phase-inverted from the main beam by 180° during the traversing of a track. By determining a difference between a tracking error signal obtained from the main beam and the tracking error signal obtained from the sub beam, a tracking error signal of a differential push-pull method is obtained.
In this case, a width W0 of the region 122 is desirably set at about 10% to 30% of the diameter of the light beam 130 on the diffraction grating 102. The provision of the region 122 can suppress an amplitude change of the tracking error signal after an operation even when the objective lens 106 moves in the track traversing direction and thus a deviation of position relationship between the regions of the diffraction grating 102 and a light beam occurs. Particularly, the effect is enhanced when the pitch of the grooves formed on the optical disk 107 is large as compared with an NA and a wavelength as on a DVD-RAM. On a DVD-RAM, information is allocated with a 0.615-μm pitch and land-and-groove recording is performed, so that the grooves where the tracking error signal is generated are each spaced at 1.23 μm from a land to the subsequent land. In this case, λ/NA<Tp is established where a wavelength λ is 660 nm and an NA is 0.65. In the case of a DVD-R and a CD, λ/NA≧Tp is established.
FIG. 21 shows an example of Japanese Patent No. 3661694 (for example, see FIG. 8).
A diffraction grating 140 indicates a diffraction grating where regions are changed at regular intervals. Regions 141 to 144 all have grooves at uniform intervals P but have diffraction gratings of different phases. The region 141 and the region 143 are in phase with each other, the region 142 and the region 144 are in phase with each other, and the regions 142 and 144 have a phase difference of 180°, from the regions 141 and 143. The regions 141 to 144 are arranged with equal widths L at regular intervals and each region has a width W1 expressed as W1=λ·D/(2NA·Tp) where λ is the wavelength of a light beam, D is the diameter of a projection 150 of the light beam from an objective lens, NA is the numerical aperture of the objective lens, and Tp is a groove pitch on the information layer of an optical disk.
Thus even when the objective lens moves in the track traversing direction, a phase difference is always 180° on an overlapping part of ±first-order diffracted light and zeroth-order light, the overlapping part being caused by grooves on a disk. It is therefore possible to suppress an amplitude change of a tracking error signal after an operation performed by the in-line differential push-pull method.